The present invention relates to rotary drill bits with a plurality of rotary cutters, each of the cutters being supported on a trunnion connected in at least one radial bearing and on a conical axial roller bearing.
Rotary drill bits are not new per se. In the prior art it is known to mount the rotary cutters on two cylindrical roller bearings which each serve as radial bearings and a conical axial roller bearing between the radial bearings. The conical rollers of the axial bearing run, in this instance, between two conical bearing races which are relatively difficult to manufacture accurately with respect to angle and alignment with each other and which are incorporated on the trunnion or in the rotary drill bit. This prior known rotary drill bit has the further disadvantage that play in the radial bearing races causes an opposite radial motion of the two bearing races of the conical axial roller bearing during rotation of the rotary cutter. This motion can lead to harmful jamming of the conical rollers between the raceways so that a premature breakdown of the axial roller bearing results. Furthermore, very little radial play must be allowed from the start in the radial bearings. Hence, the manufacturing tolerances for the bearing elements must be held within close, narrow limits which translates into relatively high manufacturing costs.